As long as you have spent time with your llama and formed a bond, teaching them to kush is usually pretty simple. With both of my llamas I have just had to crouch down in front of them while bring their heads to the ground and with a bit of patience they have gone down. Llamas very much copy what you do, so crouching down is actually a very important step. If, after a good amount of time, they have not kushed you can pinch and apply some pressure in their withers (shoulder) area and that will usually help them figure out what you are asking. If you hit the right spot it is uncomfortable for them and they will instinctively lie down.
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After you have practiced this many times, and they are very aware of what the word “kush” means, you can slowly begin to ask for a kush while standing in a more upright position. Eventually you should be able to stand straight up and have them kush for you.
Here is a baby named Durango that I have been working with showing off his kush when he was just 4 months old.